Turbochargers drive a compressor to deliver air at high density to the engine intake, allowing more fuel to be combusted, thus boosting the engine's horsepower. To do this, the rotating assembly of the turbocharger, comprising turbine wheel, compressor wheel, and shaft, may rotate at 80,000 RPM to 300,000 RPM. It is critical that the rotating assembly be balanced with a high degree of precision to improve efficiency and prevent premature wear.
The turbine wheel is materially fused to the shaft to make a shaft-and-wheel assembly. The shaft-and-wheel assembly is finished as a very accurately machined, unitary component with shaft diameters ground to tolerances in the 2.5 micron regime; thus, its inherent balance is quite good.
The compressor wheel, on the other hand, is an extremely difficult part to machine and balance. First, it is extremely critical to machine the bore (27) through the center of the compressor wheel such that it is centered on the hub at both the nose end face (21) and the hub end (22). Further, it is critical that the nut that secures the compressor to the shaft not introduce imbalance. While the nut is a relatively low mass item, for example, 6.3 gm in one case, its contribution to unbalance (as against balance) can be very large. The compressor nut must apply a heavy clamp load to the compressor wheel such that it will not rotate under any dynamic conditions, e.g., from accelerating from cold start to maximum speed, to decelerating from maximum speed at hot shutdown. The act of rotating the nut against the face (21), on the nose of the compressor wheel, can cause the nut to dig into the face and track off center, particularly when the nut is steel and the compressor wheel is aluminum. This tracking causes the mass center of the nut to move off the turbocharger axis, which results in an unbalance (N), equal to the mass of the nut times the displacement (Rn), perpendicular to the turbocharger axis.
The lower face (31) of the nut in contact with the compressor wheel must thus be manufactured to a very tight perpendicularity tolerance to the bore of the thread in the compressor nut, in the range of 0.03 to 0.04 mm, so that when the nut is threaded onto the shaft, and clamp load applied, the aforementioned lower face of the nut is applying a load close to normal to the face (21) on the nose of the compressor wheel. Failure to apply this load normal to the face of the compressor wheel will cause bending of the shaft, with the result that the mass of the compressor wheel, nut, and stub shaft will be displaced from the turbocharger axis (35) causing a large unbalance in the rotating assembly. Since this imbalance did not exist prior to tightening of the nut, it is known as “created imbalance”.
U.S. Pat. No. 4,872,817 (De Kruif) teaches that when securing a compressor wheel to a shaft, each mounting face has the potential of being out of square with respect to the axis of the shaft. This suggests the engineering solution of introducing a washer, particularly of a metal harder than that of the compressor wheel. However, since a washer has two mounting faces, it can contribute doubly to the probability of created imbalance. Tightening of the nut then causes bending of the shaft member, thereby destroying any balance the assembly may have had when originally balanced. De Kruif recommends, instead of a washer, forming the rotor member with an annular groove about the nose portion at a point near its mounting face. During tightening of the nut, in the event of uneven force application, the remaining weakened flange-like lip gives way and crimps before any shaft bending occurs, thereby eliminating any bent-shaft induced rotor-shaft imbalance. However, this solution reduces the clamping load available to restrain the compressor wheel, which should preferably be quite high.
The goal of a turbocharger manufacturer is to offer product at the lowest cost, with the highest possible reliability and durability. Balance is a key factor in the durability and reliability facets.